Being pregnant alone is not a reason for denial. There are underlying intent issues that could come into play though. For example, if your wife is entering the U.S. pregnant at 9 months there could be an intent to have a U.S. born child and eventually become a resident. When you enter the U.S. on a B1/B2 tourist visa you cannot have the intent to become a resident. As long as the officer at customs does not feel like you have this dual-intent it should not be a problem. Make sure to buy a round-trip airline ticket as proof!